DS (Direct Sequence)--CDMA is a system in which a plurality of users carry out communications using a same frequency band, and each user is identified by a spreading code. As a spreading code for each user, a spreading code such as Gold code is used. Interference signal power of another user is a reciprocal of average spreading factor (PG) in the despreading process of a receiver. However, each user, especially under asynchronous environment in ascendant mobile communications, is subject to momentary variation, short section variation, and distance variation due to independent fading.
Therefore, to satisfy a predetermined reception quality determined by the system by each user at the receiving side, it is necessary to control the transmission power to achieve a constant SIR (Signal-to-Interference Ratio) in the receiver input at the base station. Here, SIR is a ratio of the reception signal power at the user of the desired wave to the interference signal power received from another user. However, even though the transmission power control is perfect, and the SIR in the base station receiver input is maintained at a constant value, under multipath environment of mobile communications, spreading codes will never quadrate completely with each other. Therefore, the user is subject to interference due to cross-correlation of the power of a reciprocal of spreading factor at an average per one of other users.
As shown above, since the interference signal level increases with increasing number of users communicating in the same frequency band, to increase the user capacity per cell, an interference canceling technique to reduce interference from other users is required.
As interference canceling techniques, a multi-user type interference canceler and a single user type interference canceler are known. The multi-user type interference canceler not only demodulates a desired wave signal of its own channel, but also demodulates a signal of another user using spreading code information and reception signal timing of the other user. The single user type interference canceler, on the other hand, uses only the spreading code of own channel to minimize an average cross-correlation and noise component from the other user.
The multi-user type canceler includes a linear processing type (decorrelator or the like) and a nonlinear processing type. The decorrelator calculates mutual correlation of the spreading code of own channel and all other spreading codes of receiver input to determine an inverse matrix composed of the cross-correlation, and the cross-correlation is canceled by compensating for the output signal of a matched filter using this inverse matrix. Where K is a number of users, and Lk is a number of reception paths to individual users, dimension Dm of the decorrelator matrix is given by the following equation. ##EQU1##
Therefore, realization of the above technique becomes difficult as the number of users increases, which increases the circuit scale.
A nonlinear multi-user type interference canceler is a replica reproduction type interference canceler. This canceler demodulates interference signal from other user's channel, decides it to reproduce transmission information data replica, calculates an interference signal replica of each channel from this replica, and subtracts the interference replica from the reception signal, thereby demodulating the desired wave signal with enhanced SIR.
FIG. 1 shows a replica reproduction type multi-stage interference canceler (serial interference canceler) proposed in the document "Serial interference cancellation method for CDMA", IEE, Electronics Letters Vol. 30, No. 19, pp. 1581-1582, Sept. 1994.
In FIG. 1, the numeral 11 indicates a spread signal, 12, 16 are delay units, 13, 17 are matched filters, 14, 18 are respreaders, and 15 is a interference subtractor. The serial canceler comprises interference canceling blocks in a plurality of stages, connected in series, whereby the interference canceling blocks of individual stages carry out demodulation and generation of interference signal replica by turns to M users to be demodulated.
The receiver first rearranges the reception signals in the order of reception signal level. For explanation, serial numbers from 1 to M are assigned to the rearranged signals, number 1 being assigned to the highest reception signal level. The interference canceling block of the first stage makes despreading, demodulation and data decision by the matched filter 13 on the reception signal of number 1, and the resulting reproduction data is referred to as D.sub.1.sup.(1). The respreader 14 calculates an interference signal replica S.sub.1.sup.(1) of this channel from the reproduction data D.sub.1.sup.(1). The interference subtractor 15 subtracts the interference signal replica from a reception signal S passed through the delay unit 16. The matched filter 17 makes despreading, demodulation and data decision on the signal obtained by the subtraction using the spreading code replica of user 2 to obtain a reproduction data D.sub.2.sup.(1) of user 2. The matched filter input signal of user 2 is improved in SIR to the extent that the interference signal replica S.sub.1.sup.(1) of user 1 is subtracted as compared with direct despreading from the reception signal S.
Similarly, to user 2, an interference signal replica S.sub.2.sup.(1) is obtained from the reproduction data. A matched filter input signal of user 3 is obtained by subtracting interference signal replicas of users 1 and 2 from the reception signal S passed through the delay unit. Using this procedure, for subsequent users, the reception SIR can be further enhanced. When despreading the reception signal of M'th user, interference signal replicas S.sub.1.sup.(1) +S.sub.2.sup.(1) +. . . S.sub.M-1.sup.(1) of a total of (M-1) users are subtracted from the reception signal S to produce a signal, thereby considerably improving the SIR over the reception signal S. As a result, demodulated signal of M'th channel is improved in reliability.
Using interference signal replicas S.sub.1.sup.(1), S.sub.2.sup.(1), . . . , S.sub.M-1.sup.(1) of individual users estimated in the first stage interference canceling block, similar despreading, demodulation, data decision, and respreading are carried out in the second stage interference canceling block. For user 1, interference signal replicas S.sub.2.sup.(1) +S.sub.3.sup.(1) +. . . +S.sub.M.sup.(1) other than of user 1 determined by the first stage interference canceling block are subtracted from the reception signal S to produce a signal of improved SIR, and on this signal, despreading, demodulation and data decision are carried out. To other channels, similar processing is applied. That is, a signal, obtained by subtracting interference signal replicas in the first stage of channels other than own channel from the reception signal S, is subjected to respreading, demodulation, and data decision, and from the reproduction data, interference signal replicas S.sub.1.sup.(2), S.sub.2.sup.(2), . . . , S.sub.M.sup.(2) of individual channels in the second stage interference canceling block are determined.
Accuracy of the interference signal replicas in the second stage interference signal canceling block is improved compared with the interference signal replicas in the previous stage. This is because data reproduction is made based on the signal obtained by subtraction of interference signal replicas in the previous stage. By repeating serial interference cancellation in several stages, reliability of the reproduction data can be improved even further.
Under mobile communication environment, amplitude variation and phase variation occur due to Rayleigh fading in association with variation in relative positions between the mobile station and base station. In the multi-stage type interference canceler (serial interference canceler) shown in FIG. 1, it is necessary to estimate the phase and amplitude variations in the process of generating the interference signal replicas. The channel (phase; amplitude) estimation accuracy greatly affects the reception characteristics of the multi-stage type interference canceler, but realizability thereof is not described in the above document. As a method in which estimation of transmission path variation under mobile communication environment is added to the serial interference canceler of the above document, there is another document: Fukazawa et al., "Construction and characteristics of interference canceler according to transmission path estimation using a pilot signal", Proceedings of the Electronic Information Communication Society, Vol. J77-B-II No. 11, pp. 628-640, Nov. 1994.
FIGS. 2A and 2B are block diagrams showing a serial canceler shown in this document. FIG. 3 shows the channel structure of the method.
In FIGS. 2A and 2B, the numeral 21 indicates a spreading code input terminal, 22 is a first stage reproduction data output terminal of user 1, 23 is a delay unit, 24 is a pilot channel transmission path variation estimator, 25 is an interference subtractor, 26 is a first stage interference canceling block, 27 is a second stage interference canceling block, 28 is a matched filter, 29 is a transmission path compensator, 30 is a RAKE combiner, 31 is data decision block, 32 is a signal distributor, 33 is a transmission path variation adder, and 34 is a respreader.
This system, as shown in FIG. 3, is provided with a pilot channel having a known transmission pattern parallel with the communication channel. Transmission path estimation is made based on the reception phase of the pilot channel. Further, amplitude/phase estimation of the reception signal of each path of each user is carried out based on the transmission path estimation of the pilot channel. Still further, using the amplitude/phase estimation value, interference canceling of several stages is carried out by the serial interference canceling block to reproduce data of each user. In this case, as in the previous document, individual paths are ranked in the decreasing order of the sum of reception signal power. In the case of FIGS. 2A and 2B, the user 1 reception signal power is assumed as to be the highest.
In the first stage interference canceling block, demodulation is first carried out on user 1. That is, each path of user 1 is despread by a matched filter 28, in a transmission path variation compensator 29, each path of user 1 is compensated for phase variation according to the phase variation of each path estimated with respect to the pilot channel. Further, in the RAKE combiner 30, signals of the phase variation compensated paths are phase synthesized by a reception complex envelope curve of individual paths. The phase synthesized signal is decided by the data decision block 31 to obtain reproduction data of user 1. The distributor 32 distributes the reproduction data replica according to weighting at the RAKE combining, the transmission path variation adder 33 gives a phase variation of each path, and the respreader 34 makes respreading by spreading code of each path to produce the interference signal replica S.sub.1.sup.(1).
For user 2, the following processing is made. First, a delay unit 35 delays the reception signal S. The interference subtractor 25 subtracts the interference signal replica S.sub.1.sup.(1) of user 1 from the delayed signal. The first stage interference canceling block of user 2 carries out despreading, phase compensation, RAKE combining, data decision, and production of interference signal replica for each path to the output signal of the interference subtractor 25. In this case, the input signal of the interference signal canceling block of user 2 is improved in reception SIR to the extent that the user 1 interference signal replicas are subtracted. Similarly, reproduction data is estimated for each user by the first stage interference canceling block up to user M to obtain interference signal replicas.
The interference signal canceling block of second stage carries out similar processing using interference signal replicas S.sub.1.sup.(1), S.sub.2.sup.(1), . . . , S.sub.M.sup.(1) obtained by the interference signal canceling block of the first stage. For example, the second stage interference signal canceling block 27 (comprising the components 28-34 of the first stage) of user 1 makes data demodulation by despreading the signal obtained by subtracting the channel interference signal replicas other than own channel from the reception signal S delayed by delay unit 23.
A difference of the prior art method from the method described in the previous document is the following point. In the previous method, for user 2, for example, interference signal replicas S.sub.1.sup.(1) +S.sub.3.sup.(1) +. . . +S.sub.M.sup.(1) in the foregoing stage are used as interference signal replicas of all paths. On the other hand, in the method of this document, S.sub.1.sup.(2) is used as an interference signal replica of user 1 in the second stage. Compared with the estimated value S.sub.1.sup.(1) in the foregoing stage, the estimated value S.sub.1.sup.(2) in this stage is higher in reliability. Therefore, the accuracy of the desired wave signal obtained by subtracting the interference replicas and reliability of decision data obtained by demodulation are also improved.
However, in this method, a pilot channel is provided in parallel with the communication channel for each user, and a channel estimated in the pilot channel is used in each stage of interference canceling block. In this case, since channel estimation in the pilot channel is carried out independent of the interference canceling loop, to estimate channel (phase, amplitude) variation in high accuracy, it has been necessary to make averaging over a very long time (using many pilot symbols). For averaging using such numerous pilot symbols, it is assumed that channel estimation values in this period be approximately constant, therefore, it is limited to be applied to an environment of fast channel variation (high fading frequency). When fading is fast, averaging is possible only in a range where the values can be regarded as constant, it is therefore impossible to obtain a sufficient channel estimation accuracy if the number of averaging symbols is small.